Prepare the 2.2.0.dev1 release.

[benjyw]

Internal-only changes:

• Add Toolchain plugin (disabled). (#11287)

• Fix one graph test, and disable another that has been flaky for a while. (#11284)

• Make python coverage interpreter constraints match the python-setup interpreter constraints. (#11273)

• Tweaks to changelog generator script. (#11280)

• Revert "Move graph cycle detection to rust. (Move TransitiveTarget cycle detection to rust. #11202)" (#11272)

• Ignore envrc file to ease usage of direnv. (#11267)

• Use a bandit version that works with older python versions. (#11268)

• Update the MyPy requirement sync comment. (#11266)

• Use annotations import to avoid string type annotations (#11214)

• Fix rule_runner.py not being included in pantsbuild.pants.testutil (#11257)

• Rewrite fs_test.py using RuleRunner (#11248)

• Upgrade setproctitle to 1.2 (#11242)

• Prepare 2.1.1rc2. (#11286)

• Prepare 2.0.1rc4. (#11285)

[ci skip-rust]

[coveralls]

Coverage remained the same at 0.0% when pulling f370a96 on benjyw:release_2.2.0.dev1 into bde37bd on pantsbuild:master.